Col. Alexander Litvinenko, an outspoken former KGB and Federal Security Bureau agent, "has some symptoms consistent with thallium poisoning and he's also got symptoms consistent with some other type of poisoning, so it's not a hundred percent thallium," Dr. John Henry, a clinical toxicologist at University College Hospital, told reporters.

"It could be radioactive thallium," he said, adding that Litvinenko may require a bone marrow transplant.

Thallium is a colorless, odorless and water-soluble heavy metal, and can be deadly in even tiny doses of as little as one gram.

Litvinenko was under armed guard at the London hospital, the victim of what his friends and fellow dissidents called an assassination attempt by the Russian government.

Underlining the political sensitivity of the case, Henry lost his temper when reporters asked if his patient was deliberately poisoned. "I'm not a politician, I'm a doctor!" Henry shouted at one point.

Henry said Litvinenko was able to eat and to talk. "At the moment he's not getting better, but he's holding up," he said.

The Metropolitan Police said Monday that its counterterrorism unit had taken over the investigation.

The Kremlin and Russia's security agency have strongly denied any involvement.

Henry said it may be too late to determine what poison other than thallium was involved because the substance may have already degraded.

"If it's a radioactive poison with a short half-life, it may have gone. Radioactive thallium degrades very rapidly, so that by now we've missed the chance," Henry said.

Thallium is widely used in hospitals, "but not in massive doses," he said.

Although refusing to speculate about whether Litvinenko was deliberately poisoned, Henry explained that "poisons can be taken by mouth, they can be injected, they can be inhaled."

"In this case, his symptoms are gastrointestinal to start with. He had gut problems, so the probability is that he swallowed something that was poisonous," Henry said.

"Radioactive thallium adds a new dimension to this case. It means that his bone marrow is at very high risk. And we have to see how his cells recover," he added, reports AP.

"It's very difficult to treat because you have to rely on the body's natural resilience. He may need a bone marrow transplant to get him better."

"Radioactivity damages cells, and so cells begin to die," Henry added. "The cells that are affected most are in the gut and in the blood, and that is why his blood counts have gone down to zero."

In response to the unlawful December 1 arrest and detention of Chinese tech giant Huawei's chief financial officer Sabrina Meng Wanzhou by Canadian authorities in Vancouver at the behest of the Trump regime, facing possible unacceptable extradition to the US, Beijing warned its high-tech personnel last month against traveling to America unless it's essential.